Magnetic resonance imaging (MRI) is a major imaging technique used in medicine. MRI is capable of generating detailed images of soft tissues such as the brain, muscles and kidneys. Specific properties of the various compounds found inside tissues, such as water and/or fat, are used to generate images. When subjected to a strong magnetic field, the vector sum of the nuclear magnetic moments of a large number of atoms possessing a nuclear spin angular momentum, such as hydrogen, which is abundant in water and fat, will produce a net magnetic moment in alignment with the externally applied field. The resultant net magnetic moment can furthermore precess with a well-defined frequency that is proportional to the applied magnetic field. After excitation by radio frequency pulses, the net magnetization will generate a signal that can be detected.
Various electromagnets are integral parts of an MRI system. They allow the generation of the main magnetic field, the spatial encoding of the detected signals for the formation of spatial images, and correction of any irregularities. Electromagnets perform this function by generating magnetic fields with predetermined shapes. For example, the main magnet is designed to generate a magnetic field that is as uniform as possible, across all dimensions. Gradient coils on the other hand are designed to generate magnetic fields that vary linearly with a constant tangent along the three perpendicular axis of the MRI systems' imaging volume.
Manufacturing electromagnets which can generate magnetic fields with the desired requirements such as magnetic field shapes can present challenges. Specifically, to function properly, electromagnets are typically produced to operate in accordance with additional requirements besides magnetic field shape. For example, it is desirable to produce gradient coils, which when energized produce minimal eddy fields. This requirement is in addition to the linearity of the magnetic field produced. However, when the gradient coils are asymmetric in the longitudinal direction (z), for example, eddy fields and net torque and force can be generated which can disrupt the operation of an MRI system. Thus, improved electro-magnet manufacturing and operating techniques are needed to allow the construction of electro-magnets that better meet desired requirements while being able to generate magnetic fields that correspond to a desired field shape.